The present invention relates to a method for the preparation of sodium tungstate by the oxidation of hard metal scrap and/or heavy metal scrap in a molten salt.
Methods for the recovery of useful materials from ("working up of") hard metal scrap (WC--Co or WC--Co--TaC--TiC) and heavy metal scrap (W--Cu--Ni--Fe) are described in numerous patent publications. Known methods include the oxidation of hard metal scrap at elevated temperatures (U.S. Pat. No. 3,887,680), treatment with liquid zinc (U.S. Pat. No. 3,595,484) or the anodic dissolution of contained binder materials (AT-A 380 495). These methods are either expensive as regards apparatus or are suitable only for certain types of scrap.
Moreover the reaction of hard metal scrap in molten salts is important industrially.
Thus U.S. Pat. No. 4,603,043 describes the working up of W-containing materials in a molten salt consisting of NaNO.sub.3 and NaOH at temperatures of 500 to 700.degree. C. Here, in order better to control the reaction, NaOH and the W-containing component are first of all heated to 560 to 600.degree. C. The NaNO3 required as oxidising agent is subsequently added over a period of 2 to 3 hours.
According to DE 3144295, the working up of hard metal scrap is likewise carried out in an alkali hydroxide/alkali nitrate melt, in a molten salt consisting of 40 to 80% alkali nitrate and 20 to 60% alkali hydroxide. 5% of NaCl is added in order to lower the melting point. The melting temperature is 550.degree. C. The melt is subsequently poured into water.
DD-A 207 932 discloses the working up of hard metal scrap in pure sodium nitrate melts or pure sodium nitrite melts. Here the stoichiometric excess of the alkali component is to be 10 to 15% and the melting temperature 900.degree. C. The reacted melt is cooled to room temperature prior to being dissolved in water.
According to IN-A 157 146, a molten salt consisting of alkali hydroxide, preferably KOH or NaOH, and an alkali nitrate as oxidising agent, preferably KNO.sub.3 or NaNO.sub.3, is employed for the working up of hard metal scrap. The melting temperature varies between 350 and 460.degree. C. In this connection it was established that at 400 to 420.degree. C. the yields of tungsten are 90 to 94%. At 440 to 460.degree. C. the yield of tungsten can be increased to 99%. In this case the reaction can be controlled only with difficulty. In addition emission of nitrogen oxides occurs.
The disadvantage of all reactions in nitrate and/or nitrite media is that the highly exothermic reaction is difficult to control. Consequently the overall reaction is considered to be problematic as regards industrial safety. In addition there is a frequently uncontrollable emission of nitrous gases. Nitrite and nitrate contained in the sodium tungstate solution and in the filtration residues render working up extremely difficult when a solvent extraction has to be subsequently carried out.
The object of the present invention, therefore, is to provide a method for the working up of lumps of hard metal scrap and heavy metal scrap, which does not have the disadvantages described.